A wide range of implantable medical devices are provided for surgical implantation within patients, such as cardiac pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization therapy (CRT) devices or other implantable cardiac rhythm management devices (CRMDs.) Typically, such devices are equipped with one more leads for sensing electrical signals such as cardiac signals. However problems can arise if an insulation breach occurs in the lead, potentially exposing one of its internal conducting wires to blood or other patient tissues, yielding a form of short circuit that interferes with the capability of the lead to properly sense cardiac signals and delivery appropriate therapeutic pulses, as well as resulting false sensing leading to impropriate therapy. For example, some bipolar leads include a central wire for connecting a first electrical terminal of the CRMD to a tip electrode at the distal end of the lead. An outer coaxial conductor is provided for connecting a second electrical terminal (of opposite polarity) to a ring electrode, which is mounted near the tip electrode. An outer insulating sheath surrounds the ring conductor to isolate it from blood and other patient tissues. In use, however, the sheath can abrade, eventually exposing the ring conductor to patient tissues. Similar problems can occur in “flat wire” leads as well. Abrasion or other lead failures can cause: impropriate therapy due to false over-sensing; withheld therapy due to under-sensing or over-sensing (inhibition); failure to deliver therapy due to due to conductor failures; damage to the device itself, etc. Long term reliability of CRMD leads is a key factor clinicians consider when selecting particular cardiac systems for implant within patients.
Although lead reliability has improved dramatically over the years, leads can still fail for various reasons. Accordingly it would be desirable to provide techniques for early detection of the lead failures so that leads can be replaced before complete failure occurs. This would provide valuable protection to patients and additional assurances to the healthcare providers. Typically, to detect a lead failure using a CRMD system with a bipolar lead, impedances are measured between tip and ring electrodes of the bipolar lead. If the bipolar impedance drifts out of range or fluctuates more than expected, such indicates the lead may be experiencing a fracture. However, this technique may not be sensitive enough for early detection of lead integrity issues arising due to insulation breach because the impedance change may still be relatively small.